The present invention relates to a next-generation energy source, in particular, relates to a method for producing hydrogen gas, which is expected to use as fuel for fuel cell, through pyrolysis of water.
Since only water is generated through combustion of hydrogen, hydrogen has a feature as a high-quality energy source from the viewpoint of the physical and the chemical properties thereof. Hydrogen also attracts an attention as a clean medium with regards to the problems of the environment, and it is expected that hydrogen will increasingly be demanded as an energy source for the future.
However, in our country (Japan), there is not yet completed a hydrogen producing system that is low-cost and has a high efficiency.
As a conventional method for producing hydrogen, there are a method through electrolysis of water and a method through a thermochemical cycle. With regards to the method through electrolysis, the conventional process is improved and a new process is developed to advance the energy efficiency. However, such a method is not considered to be effective in the total balance due to the limit of electric power in our country. With regards to the method through a thermochemical cycle, various methods are proposed and implemented; however, they still have problems as follows:
[Steam Reforming Method]
This is a method of obtaining hydrogen by reacting methane gas and steam which is heated up to 700xc2x0 C.-800xc2x0 C.
This method has drawbacks that it requires a high temperature of reaction, generates carbon dioxide which causes the global warming, and requires large-scale facilities.
[Conversion Reaction of Carbon Monoxide]
CO+H2O=CO2+H2 
The above-mentioned conversion reaction is conducted using iron oxide (Fe3O4) or a catalyst in a group of zinc oxide-copper. Also, M. Laniecki, et al. reports that NaY-type zeolite can be used as a catalyst.
The method using conversion reaction of carbon monoxide has drawbacks that it requires a high temperature of reaction and generates carbon dioxide, too.
[Direct Decomposition of Water Through Triiron Tetroxide (Fe3O4)]
This method, which was tested by New Energy and Industrial Technology Development Organization (Public Corporation of Japan), comprises eight (8) processes of an iron-steam group, as shown in FIG. 2. This method has drawbacks that it requires a high temperature for producing FeO through deoxidation of Fe3O4 and that the apparatus thereof is complicated due to the combination of multistage reactions.
[Cycle of Halogen Group]
FIG. 3 shows a cycle for producing hydrogen gas, which is referred to as xe2x80x9cUT-3xe2x80x9d in Tokyo University. It is comprised of the following multistage reactions:
This method also has drawbacks that it requires a high temperature of reaction and that the apparatus thereof is complicated due to the combination of multistage reactions.
[Iron-Bromine Cycle]
A cycle for producing hydrogen gas is conducted by Osaka National Research Institute(Public Corporation of Japan), using the following equations:
This method also has drawbacks that it requires a high temperature of reaction and that the apparatus thereof is complicated due to the combination of multistage reactions.
[Oxide Cycle]
A cycle for producing hydrogen gas is conducted by Los Alamos National Laboratory, and it is reported that it proceeds up to 40 cycles using the following equations:
Since this method uses the complex oxide of strontium and uranium, it is inferior from the viewpoint of natural resources, and also there is fear that it would cause environmental pollution.
[Sulfur Group Cycle]
This is a cycle for producing hydrogen gas by combining the following multistage reactions; however, it is doubtful whether an actual experiment is conducted or not:
As mentioned above, any one of the conventional technologies with relation to producing hydrogen, except for the method through electrolysis, requires a high temperature of reaction, the apparatus thereof is complicated and large-scale due to the combination of multistage reactions, and it generates the reaction product such as CO2 or the like. Therefore, the present inventors have studied a hydrogen gas producing method, is which is low-cost, has a high efficiency, does not generate the reaction product such as CO2 or the like.
According to the oxide cycle by Los Alamos National Laboratory, the production of hydrogen is attempted at the temperature less than or equal to 600xc2x0 C. by using the complex oxides of strontium and uranium and the pyrolysis reaction between strontium and hydroxide. The present inventors conceived an idea that hydrogen gas can be produced using the oxide cycle with regards to zeolite comprising the complex oxide of silicon and aluminum. As a result, as proposed by PCT international publication No. WO98/51612, hydrogen gas can be produced at a low temperature and through only one stage reaction by conducting direct pyrolysis of water with a zeolite catalyst.
However, in the above-mentioned method using a zeolite catalyst, there are cases in which hydrogen gas of high concentration can be produced, however it is still impossible to stably produce hydrogen gas of high concentration.
According to the present invention, there is provided a method for producing hydrogen gas, comprising the steps of: granulating zeolite; applying water to the zeolite at the temperature equal to or greater than 300xc2x0 C. and less than or equal to 500xc2x0 C.; and conducting direct pyrolysis of water using the zeolite as a catalyst. The granulation of zeolite enables to stably produce hydrogen gas.
Zeolite is crystalline aluminosilicate, and mainly composed of SiO2, Al2O3, H2O, Na2O, K2O and CaO. Zeolite has various structures. Also, the chemical composition thereof has variety because the Si/Al ratio in the framework, the ratio of exchange cation, and further the number of water molecules within a unit cell are unstable. Therefore, at the present time, zeolite has 40 kinds or more as a natural mineral, and also 150 kinds or more as synthetic zeolite.
In order to produce hydrogen at high concentration, alkali metal halide and/or alkaline-earth metal halide is/are added to zeolite at the time of granulating the zeolite.
As a halogen element of which the above-mentioned alkali metal halide and/or alkaline-earth metal halide is/are comprised, Cl (chlorine), Br (bromine), F (fluorine) or the like can be listed. Cl is the most effective. It is assumed that the reason why Cl is the most effective is that chloraquo complex can be easily formed with water molecules.
As an alkali metal, Na (sodium), Li (lithium), K (potassium) or the like can be listed. As an alkaline-earth metal, Mg can be listed. Among these, Li is the most effective. The reason is assumed as follows:
Li has the smallest ion radius and it is easy to exchange ions between metals, such as Na, Mg, K or the like, which are included in natural zeolite. As a result of this, the density of the electric charge of zeolite is increased, the dissociation of water is accelerated, the pressure equilibrium constant is made large, and thereby the decomposition of water is accelerated.
Further, it is preferable that the addition rate of alkali metal halide and/or alkaline alkaline-earth metal halide be from 1 to 20g with respect to 100 g of zeolite.
Furthermore, pyrolysis under a depressurized condition improves the yield thereof.